Frans - instead of bio-balls - why don't you think of perhaps rather using zeolite stones?

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JB , I will be using them in my cryptic zone as well (see there is a lot of discussion about this at the mo). As my system is basically FO (holding tanks) I am setting up a filter system (which I HOPE) incorporating all aspects ie mechanical , wet / dry tricle, cryptic, macro algea, skimmer